Bomb release radio system



2 Sheets-Sheet 1 v VVVV R E a E K 0 E BOMB RELEASE RADIO SYSTEM FiledNov. 1s, 194$ -IHW- INVENTOR Eugene a Ki'zer am ATTORNEY Patented Apr.27, 1948 2,440,609 r norm aEEEAsE more SYSTEM Eugene 0.

Keizcr, Princeton, N. 3., assignor to Radio Corporation of America, acorporation of Delaware 1 Claim.

This invention relates to radio bomb release systems, and moreparticularly to improvements in systems of the type described incopending application Serial No. 524,794, filed on March 2, 1944, byRoyden C. Sanders, Jr., and William R. Mercer and entitled Radio bombrelease system, and which has matured into Patent No. 2,412,632 issuedDecember 17, 1946, wherein a frequency modulated signal is radiated froma mobile craft toward a selected target, received after reflection fromsaid target, and the received signal compared with the transmittedsignal to actuate a bomb release device upon the occurrence of apredetermined relationship between the target distance and the speedwith respect to the target.

Systems of the described type are sometimes subject to disturbancescaused by the absence of a desired target or caused by severe fading ofthe received signals, as a result of multiple path transmission betweenthe target and the bombing craft. Another effect which is almostinvariably present in the operation of such systems is that of seareturn, or reflection from the surface v upon which the target lies.These two effects will, on occasion, conspire to cause premature releaseduring pronounced fading conditions, since the so-called sea return willover-ride the target signal, simulating a signal from a target atminimum range. A similar efiect may also be produced by improperadjustment of the radio system.

Copending U. S. patent applications Serial No. 546,269, filed on July24, 1944, by James H. Ludwig and entitled Radio bomb release system, andSerial No. 623,366, filed on October 19, 1945, by Ben R. Cole andentitled Radio system describe systems for preventing premature releasesby simulating, during fading periods, the conditions which would appearif there were a target at maximum range. 4 While these systems operatesatisfactorily to prevent premature release, they may cause a laterelease to occur if the target reflection signal reappears shortlybefore the proper instant of release, owing to unavoidable delay in therecovery of the computer system.

It is the principal object of the present invention to rovide animproved method of and means for preventing false releases. Anotherobject is to provide a system of the described type which will recoverrapidly if the target signal returns, thus minimizing the possibility oflate releases.

These and other objects will become apparent to those skilled in theart,

the following description with reference to the accompanying drawings ofwhich upon consideration of 2 Figure 1 is a schematic diagram of a bombrelease system embodying the invention,

Figure 2 is a graph illustrating variations in frequency of signalstransmitted and received in 'the operation of the system of Figure 1,

Figures 3 and 4 are graphs of square wave switching voltages occurringin the operation of Figure 1, and I Figure 5 is a graph illustrating theswitched counter output currents produced in the opera-' tion of thesystem of Figure 1.

Refer to Figure 1. A system oi. the type described in theabove-mentioned Sanders et al. application is illustrated. A radiotransmitter l is connected to an antenna 3 and to a frequency modulator5. The modulator 5 may be of the vibratory variable capacitor type suchas that described in copending U. S. patent application Serial No.471,003, filed January 1, 1943, by S. V. Perry and entitled Capacitymodulatorunit, or any other known device for varying the frequency ofthe transmitter I in response to a modulating voltage. The input circuitof the modulator, 5 is connected to a wave shaping circuit 1 which isconnected through a voltage divider 9 to a battery H and a periodicswitch iii. The switch I3 is arranged to be operated by a cam 15 drivenby a motor ll. The motor I! is connected through a switch l9 to a powersource such as a battery 2|.

A receiving antenna 23, similar in construction to the antenna 3, isconnected to a detector 25.

The transmitter l is also connected to the detector 25 through a line24. Both antennas 3 and 23 are preferably directive, and are arranged toprovide maximum response in the same direction.

The output circuit of the detector 25 is connected to an amplifier 29,which is provided with an A.-V.-C. circuit including a rectifier 30connected to rectify a portion of the amplifier output. and a filtercomprising capacitors 26 and 21 and a resistor 28, through which therectified voltage is applied to a bias circuit of the amplifier in anyknown or conventional manner for controlling the amplifier gain. A D.-C.source comprising a battery 36 and a voltage divider 38 is connected tothe rectifier 30 and adjusted to bias it so that no rectification willoccur with less than a predetermined output from the amplifier 29. Theoutput circuit of the amplifier 29 is connected to an amplitude limiter3|. The output circuit of the limiter 3| is connected to a pair ofaveraging cycle counter circuits, generally desiga phase invertercircuit 6| to the switch l3.

nated by the reference numerals spectively. The counters 33 and 35 areprovided with a 33 and 35, re-

common load resistor 31, which is connected to D.-C. source 36. Thecathode circuit of the tube 39 includes a resistor 43 tapped at a point44. The cathode of the tube 39 is connected to the cathode of a tube 46.The control grid of the tube 46 is connected to a bias source comprisinga voltage divider 50, connected across the battery 36. A relay 45 isincluded in the anode circuit of the tube 46. Y

The counter 33 includes a capacitor 49 connected to the cathode of adiode and to the anode of the triode 53. The control grid of the triode53 is coupled to the switch l3. The cathode of the tube 53 isconnectedto the cathode of the tube 39. The anode of the diode 5| isconnected to the load resistor 31. The counter 35 comprises a capactor55 connected to the cathode of a diode 51 and to the anode of a triode59. The control grid of the triode 59 is coupled through The anode ofthe diode 51 is connected to the cathode circuit of the tube 39 at thepoint 44. The cathode of the triode 59 is connected to the control gridof the tube 39 and the upper end of the resistor 31. The lower end ofthe resistor 31 is connected to a voltage divider 63 across the battery35. The load resistor 31 is bypassed to ground by a capacitor 61.

The operation of the system thus far described is as follows: The motorl1 operates the switch l3 by means of the cam l5 to connect the batteryperiodically to the voltage divider 9, thus producing a square wavevoltage variation of the voltage across the voltage divider 9. Thesquare wave voltage is attenuated to an extent depending upon theposition of the adjustable divider 9, and is applied to circuit 1includes a low pass filter or other means for integrating the squarewave input with respect to time to produce an output of triangular waveshape. The triangular wave output of the wave shaping circuit 1 isapplied to the modulator 5, causing corresponding triangular wavevariation of the frequency of operation of the transmitter l. Thefrequency modulated signal produced by the transmitter I is radiated bythe antenna 3 to the target, not shown. Part of the energy striking thetarget is reflected to the receiving antenna 23. v

The received reflected signal is combined in the detector 25 with someof the original frequency modulated signal, which is conducted directlyfrom the transmitter I to the detector 25 through the line 24. Theoutput of the detector and lower limits I: and f1, respectively, about amean value In. The sweep width fa-fi is proportional to the amplitude ofthe triangular wave input to the modulator 5, and hence is a function ofthe position of the adjustable contact of the voltage divider 9.

The reflected signal is delayed with respect to the transmitted signalby the time required for the radiation to travel from the transmittingantenna 3 to the target, and back to the receiving antenna 23. This isindicated by the dotted line 15 in Figure 2. The reflected signal variesin frequency over the same range fz-fi as the transmitted signal, butconstantly differs in frequency from the transmitted signal by an amountproportional to the distance. This difference in frequency is 2 2;cycles per second where S=fz-f1 in megacycles per second. fm is themodulation frequency in cycles per second, or frequency of operation ofthe switch l3, and d is the slant distance in feet. If the equipment ismoving toward the target, the received signal is increased in frequency,owing to Doppler effect, by an amount cycles per second during increasein frequency of the transmitted signal, and

cycles per second during decrease in frequency of the transmittedsignal.

equal to the difference between the instantaneous frequency of thetransmitted and received signals. The beat output of the detector 25 isamplified by the amplifier 29 and limited to a constant amplitude by thelimiter 3|. The output of the limiter 3| is a square wave voltage havinga frequency equal to the difference between the frequency of thetransmitted and received signals and a constant amplitude, E8.

Referring to Figure 2, the frequency of the transmitted signal isrepresented by the solid line 13. This frequency varies throughout themodulation cycle in response to the triangular wave output of the waveshaping circuit 1, between upper The constant amplitude beat frequencyoutput of the limiter 3| is applied to both of the counters 33 and 35.The connections to the modulator 5 are such that during the modulationdownsweep, or decrease in frequency of the transmitted signal, theswitch I3 is closed, applying a positive pulse to the control grid ofthe triode 53 and to the phase inverter 3|, as indicated by the graph ofFigure 3. The phase inverter 6| provides a negative pulse which isapplied to the control grid of the triode 59 of the counter 35, asindicated by the graph of Figure 4. The triode 59 is thereby cut oil,and the counter 35 prevented from operating.

During negative half cycles of the limiter output, the capacitor 49 ischarged through the diode EI and the resistor 31. The values of thecapacitor 49 and resistor 31 are such that the capacitor 49 becomessubstantially fully charged to the limiter output voltage E; during eachcycle of the limiter output. During positive half cycles, the capacitor49 is discharged through the triode 53 to the potential existing at thecathode 'of the tube 39, which is substantially equal to causes anaverage current in to flow upward through the resistor 31 as indicatedby the dash arrow. This current is proportional to the prodnot of thecharge deposited in the capacitor 69 during each cycle, and the numberof cycles per second:

id=frQ=frcilEa where C: is the capacitance of the capacitor 49.

Since During the modulation upsweep, the switch I3 is open,-providing anegative pulse at the grid of the trlode 53 and the phase inverter GI,and a positive pulse at the grid of the triode 59, as indicated by theportions 19 and ill, respectively,

of the graphs of Figures 3 and 4. The counter 33 is now inoperative andthe counter 35 operates. The triode 59 is conductive, allowing thecapacitor 55 to charge through the resistor 31 during positive halfcycles of the output of the limiter 3|. During negative half cycles ofthe limiter output, the capacitor 55 is discharged through the diode 51to the potential appearing at the tap 44 on the resistor 63, which isslightly less than the potential at the cathode of the tube 39 and hencethat of the cathode of the tube 53. Thus during the modulationupsweepflthe counter 33 causes a current iu to flow downward through theresistor 81, as indicated by the solid arrow. This current isproportional to the product of the charge deposited in the capacitor 55during each cycle, and the number Of cycles per second:

where in is the beat frequency, Q is the charge per cycle, C1 is thecapacitance of the capacitor 55, and Ea is the amplitude of the outputof the Refer to Figure 5, wherein I1 is the average component of currentduring upsweep due to distance, I: is the average component of currentduring downsweep due to distance, I: is the resultant average componentof current due to distance, 14 is the increase in negative averagecomponent of current during downsweep due to speed, and Is is thedecrease in positive average current during upsweep due to speed. Theresultant average voltage across the resistor 31 is eo=ioR, where R isthe resistance of the resistor 31.

The tap of the voltage divider 63 is adjusted to apply a positivepotential of, for example, approximately 'l0 volts to the lower end ofthe resistor 31. The purpose of this arrangement is to provide asuitable operating point for the cathode follower tube 89. Denoting thisvoltage as e1, the total voltage at the control grid of the tube 38,referred to ground potential, is eo-i-er. Inasmuch as the entire load ofthe tube 39 is in the cathode circuit. the anode current will assume avalue such that the drop in said load circuit is very slightly greaterthan the voltage between the control grid and ground, and as a practicalmat ter, substantially equal to the grid voltage.

This is the case only so long as the tube 46 is non-conductive. Thevoltage divider 50 is adjusted to apply a positive voltage em to thecontrol rid of the tube 46. Since the cathode is also positive, at avoltage substantially equal to eo-I-er, the tube 46 will be cut off aslong as eo+e1 exceeds ea by more than a predetermined v l a e whichdepends upon the design Of the tube 46 and the anode voltage suppliedthereto by the battery 35. The voltage e: is adjusted in accordance withthe altitude at which a bombing run is to be made, as described indetail in the abovementioned Sanders et al. application.

Assume that the bombing craft is travelling toward the target at asubstantially constant velocity :0. Initially, the distance d isrelatively great, and hence in is relatively large. The voltage eo istherefore of maximum value, and eo+e1 greatly exceeds ea. As the targetis approached, eo gradually decreases, while e1 and e: remain constant.At a predetermined time T before the bomber is directly over the target,eo falls to a value low enough so that the tube 46 will conduct,energizing the relay to actuate the bomb release mechanism. The voltagedividers 9, 50. and 63 are adjusted as described in the above-mentionedSanders et a1. application to make the time T equal to that of thatrequired for a bomb to fall from the altitude h at which the bombing runis to be made.

Under ideal conditions, the above-described operation willcause releaseto occur at the proper instant to score a hit upon the selected target.Ordinarily the signal reflected by the target is stronger than thatreflected by the surface upon which the target is lying, so that thevoltage eo is a function of substantially only the target distame andthe speed of the bomber with respect to that of the target. However, ifthe target signal fades, surface reflections will be picked up by theapparatus. These reflections may simulate a signal from a targetdirectly under the bomber or that from a. target at a distance which ismuch less than the distance or the actual target. Acoordinglv, thevoltage eo will decrease, and may decrease to such an extent that thetube 48 is allowed to conduct, actuating the relay 45 and causingrelease of a bomb.

This difliculty, although caused by the absence of the target or byfading, is not remedied by the A.-V.-C. device, which merely tends tomaintain the general signal level, with no discrimination between targetreflection and surface refiection.

In accordance with the instant invention, the tube 32 is provided withits anode connected to the grid of the relay tube 46. The cathode of thetube 32 is connected to the tap of a voltage divider 34 across thesource 36, and the control grid is connected to the output circuit ofthe A.-V.-C. rectifier 30.

Under normal conditions of operation, as described above, relativelystrong reflection signals are received from the target, overcoming thethreshold bias derived from the voltage divider 38, so that therectifier 30 operates. This provides a negative voltage to the gaincontrol circuit of the amplifier 29 and also to the control grid of thetube 32. The voltage dividers 34 01? when the rectifier 30 operates, andtherefore has no effect upon the voltage at the grid of the tube 48.When the received signal falls below a predetermined amplitude, therectifier 30 ceases to operate andthe voltage at the grid of the tube 32decreases, causing the tube 32 to conduct. The tube 32 then presents alow resistance from anode to cathode, reducing the voltage at thecontrol grid of the tube 48. Thus so long as the received signal islower than a predetermined amplitude, the tube 46 is prevented fromconducting; however, that both the counters I3 and 38 and the cathodefollower 39 continue to operate, although the voltage at the cathode ofthe tube 39 does not correspond to the true relationship between targetrange and speed.

If the target signal returns before the bomber reaches the releasepoint, the rectifier 30 starts It should be noted,

to operate again, cutting oil the tube 32 and 1 allowing the grid of thetube 48 to resume its normal potential, causing a normal release to Ioccur. The time required for restoration of normal operating conditionsafter return of the target signal depends primarily on the constants ofthe automatic gain control filter elements 26, 21 and 28, and may bemade of the order of one twentieth second. The time constant of thecounter load circuit, comprising the resistor 31 and capacitor 61, isnecessarily much greater; however, the amount by which the voltage inthis circuit must change when the target signal reappears is not verygreat, since the counters 33 and35 both operate, although inaccurately,during fading. Thus the system recovers rapidly enough to efiect correctrelease even though the target signal reappears just prior to the propertime for release. i

the relationship between speed and range or a target and'rlay meansnormally responsive to theattainment of a predetermined magnitude bysaidvoltage to effect release. apparatus for preventing false operation ofsaid system comprising an automatic gain control circuit for saidreceiver, normally inoperative means for biasing said relay means toprevent release, and means responsive to the output of said automaticgain control circuit to operate said last-mentioned means and thus biassaid relay circuit during signal fading periods. EUGENE O. KEIZER.

REFERENCES CITED :The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,193,843 Robinson Mar. 19, 1940Host et al. Oct. 15, 1946

